A key issue for wide-range optical DC voltage sensing based on interferometric measurements is how to uniquely determine the voltage-induced electro-optic phase shift across many 2π periods. To that end, we have demonstrated in a previous patent application [1] an interference-contrast-based method, which works well for a voltage range beyond ±500 kV.
Modulation phase detection (MPD) [2] is a powerful interrogation technique to measure phase shift in an optical system. With this technique, a high-frequency phase modulation waveform is superposed onto the phase shift to be measured. This phase shift can then be derived from the measurement of the polarimetric optical response waveform. A great advantage of the MPD technique is that, because the phase shift of interest is calculated from the shape of the polarimetric response waveform rather than an absolute signal amplitude, the measurement (and particularly its zero-point stability) is in principle not affected by optical power fluctuation or loss variation. Compared to traditional polarimetric phase measurement techniques, MPD enjoys superior accuracy, DC stability and robustness.
Typically, MPD sensors are implemented in a reciprocal configuration, either in the form of a Sagnac interferometer or in a reflective form, in order to cancel phase shifts from additional birefringent elements in the system (such as PM fibers or the phase modulator crystal), which may drift slowly, e.g. with temperature change or mechanical disturbance. Such reciprocal MPD schemes are usually termed “non-reciprocal phase modulation”, because the non-reciprocal imposed phase modulation and the phase shift to be measured are the only phase shifts that remain in the measured signal. The reciprocal MPD schemes have seen wide adoption in fiber-optic gyroscopes [2], and later in fiber-optic current sensors (FOCS) [3, 4]. Recently, we have shown that a non-reciprocal form of MPD, namely differential MPD [5], can also be used to achieve similar performance, which is of great benefit for applications requiring a bulk optic sensing element.
The MPD technique can be utilized for optical voltage sensing [6], where the differential electro-optic phase shift between two orthogonally polarized light waves in a Pockels crystal is measured to determine the applied voltage. In cases where the required voltage range is larger than the n-voltage of the sensing crystal, we have shown that one may disambiguate the phase shift periods by measuring the interference contrast in addition [1], which varies as a function of the group delay (and equivalently the phase shift) between the two polarized light waves within the coherence range of a low-coherence light source. The combination of the phase shift principal value and the interference contrast allows the unambiguous determination of the electro-optic phase shift in many 2π periods, covering a voltage range >±500 kV. As an example [1], we have shown that in an open-loop sinusoidal-wave MPD scheme, the interference contrast may be determined by measuring the DC level of the optical response.
Most commercial MPD sensors use square-wave modulation with closed-loop control and the scheme taught by [1] is not applicable to such systems.